A power conversion device is mounted on various types of vehicles, e.g. on hybrid vehicles and electric vehicles so as to drive electric motors, etc. For example, an electric motor is generally equipped with semiconductor modules forming an inverter, a cooling unit through which a coolant flows, a capacitor module including a smoothing capacitor, a circuit board and a casing. The casing accommodates the semiconductor modules, the inverter, the cooling unit, the capacitor module, the circuit board, etc. In general, various attempts have been made so as to reduce an overall size of the casing. For example, an attempt has been made to ensure optimum structure and arrangement of the semiconductor modules, the capacitor module, the inverter, the cooling unit and the circuit board in the casing.